This is ad-hoc, and the spec doesn't seem to tell us what to actually do
here. Without this, following the spec steps for loading relative
`@import` URLs from a `<style>` tag always fails, because that uses the
parent style sheet's location as the base URL.
This is not really a context, but more of a set of parameters for
creating a Parser. So, treat it as such: Rename it to ParsingParams,
and store its values and methods directly in the Parser instead of
keeping the ParsingContext around.
This has a nice side-effect of not including DOM/Document.h everywhere
that needs a Parser.